This invention relates to voltage magnitude control systems of the tap changing type.
A step voltage regulator is an autotransformer provided with load ratio control equipment for regulating the voltage on the feeder or bus to which it is connected. A typical step voltage regulator may have a 100 percent exciting winding in shunt with the line on the source side and normally maintains the voltage on the load side within a desired voltage bandwidth by a 10 percent tapped buck/boost winding in series with the line. The series winding has taps connected to the stationary contacts of a tap changer dial switch having a pair of rotatable selector contacts driven by a reversible motor into sequential engagement with the stationary contacts and usually provides the ability to change the effective turns ratio from input to output plus or minus 10 percent in 32 steps of 5/8 percent voltage increments. The rotatable contacts of the tap changer switch are usually connected through slip rings to the opposite sides of a bridging centertapped autotransformer reactor, termed a preventive autotransformer, to permit transition from one tap position to another without interrupting the load current. When the selector contacts bridge adjacent stationary contacts, the high reactance of the preventive autotransformer limits circulating current to a safe value and reduces burning and erosion of the tap changer contacts and also provides a voltage midway between the physical taps to thereby provide twice the number of steps. However, such a preventive autotransformer has continuous energy losses in operation and is bulky and expensive to construct, and my copending application Ser. No. 818,987 filed July 25, 1977 discloses a tap changing voltage regulator which eliminates such preventive autotransformer and also prevents arcing at the tap changer selector contacts by providing a half-tap voltage auxiliary winding and an auxiliary switch which permits a selector contact to step arclessly to an open circuited new tap and then connects the selector contact in series with the auxiliary winding and a current-limiting inductor and the load at reduced voltage to effect a tap change without interruption of the load circuit. Although the voltage regulator disclosed in my aforementioned application provides greatly improved results over regulators using preventive autotransformers, it has the disadvantage that arcing occurs at the auxiliary switch contacts during interruption of circulating current.